Optical transceiver module

ABSTRACT

Provided is an optical transceiver module of an optical transceiver, which is used for optical communications. The optical transceiver module prevents electrical crosstalk between a light source and a light receiver. Additionally, the optical transceiver module includes an optical transceiver unit including a light source and a light receiver together integrated into a substrate, a circuit unit including a drive circuit driving the light source and a detect circuit reading a signal of the light receiver, and a crosstalk prevention unit connected between the substrate and ground to prevent electrical crosstalk between the light source and the light receiver.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This applicationThe present patent application is a Reissue of U.S. Pat.No. 7,373,032, issued on May 13, 2008, which claims the benefit ofKorean Patent Application No. 10-2005-0121240, filed on Dec. 10, 2005,in the Korean Intellectual Property Office, the disclosuredisclosures ofwhich isare incorporated herein in itstheir entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical communications, and moreparticularly, to an optical transceiver module for opticalcommunications.

2. Description of the Related Art

An optical transceiver includes a light source, a light source driver, alight receiver, a light filter, a preamplifier, etc. The opticaltransceiver is usually formed by hybrid integration of these parts.Thus, many attempts have been made to integrate the light source, thelight filter, and the light receiver on a single substrate.Additionally, many attempts have been made to integrate only the lightsource and the light receiver on a single substrate. When the parts areintegrated on a single substrate, a package cost can be reduced due tosimple packaging. In the optical transceiver module, the package cost ismore than half of the total cost of the parts. Thus, it is necessary andimportant to reduce the package cost by using single-integrated opticaldevices.

Accordingly, many attempts have been made to form the light source andthe light receiver on a single substrate.

FIG. 1 is a schematic view illustrating an optical transceiver unit inwhich a laser diode (LD) and a photo-detector (PD) are integrated in anin-line manner into a single substrate. That is, the optical transceiverunit is a device in which a light source and a light receiver areintegrated into the single substrate in an in-line manner. A portion Lindicates a light source portion corresponding to the LD. A portion Dindicates a light receiver portion corresponding to the PD. On the otherhand, a portion A indicates an absorber between the LD and the PD, whichprevents light emitted from the LD from being delivered to the PD.Waveguides or active layers 1, 2 and 3 in the LD, the absorber, and thePD are formed on the substrate along the same axis. Moreover, inputlight and output light are delivered through the waveguides 1, 2 and 3.

Feedback is achieved through a diffraction grating G. The diffractiongrating G and the active layer 1 supplying a gain make up a distributedfeedback (DFB) LD that is used as the light source. Accordingly, the LDoutputs an output signal S1. Since an input signal S2 uses a transparentwave with respect to the waveguide 1 of the LD and the waveguide 2 ofthe absorber, the input signal S2 is not absorbed into the LCD and theabsorber, and delivered to the PD. For example, a wavelength of theoutput signal S1 is 1.3 μm, and a wavelength of the input signal S2 is1.55 μm. Moreover, the active layer 1 of the LD and the active layer 2of the absorber have a bandgap wavelength of 1.3 μm, and the activelayer 3 of the PD has a bandgap wavelength of 1.55 μm. Accordingly,although the input signal S2 passes through the active layer 1 of the LDand the active layer 2 of the absorber, and then reaches the PD, theoutput signal S1 cannot reach the PD. Other contents related to theoptical transceiver unit are disclosed in OFC98, p. 350.

The single-substrate integrated optical transceiver unit disclosed abovehas the disadvantage of generating electrical crosstalk.

FIG. 2 illustrates a structure of an optical transceiver module usingthe optical transceiver unit of FIG. 1, which includes a modulestructure driving the LD and reading a signal input to the PD in thesingle-substrate integrated optical transceiver unit. The detaildescription related to FIG. 2 is disclosed in U.S. Pat. No. 6,148,015.

Referring to FIG. 2, a method of driving the LD is as follows.

An optical current generated in the absorber A, which prevents lightemitted from the LD from being absorbed into the PD, is read anddelivered to a controller 16. The controller 16 compares the opticalcurrent to an appropriate physical quantity (particularly, a voltageV1), and then supplies a direct current (DC) into the LD. The controller16 is a voltage comparator comparing voltages at both terminals andoutputting a comparison result. At this point, the absorber A serves asa monitor PD in a conventional optical transceiver unit, and the methodof driving the LD by controlling the DC drive current of the LD usingthe read current of the monitor PD is one of the conventional methods.An inductor 14 is placed in front of the controller 16, and a capacitorC_(I) placed in front of an input terminal LI delivers an alternatingcurrent (AC) that drives the LD.

Next, a method of reading a signal input to the PD is as follows. The PDis operated by supplying a reverse bias voltage. That is, the reversebias voltage is applied to the PD by applying a ground voltage to asubstrate 10 and applying a negative voltage from a negative powersource 19. An inductor 17 in front of the negative power source 19allows only a DC current to pass through. Meanwhile, a capacitor C_(O)transmits a detected AC component to an output terminal DO.

In this method, the substrate of the single-substrate integrated opticaltransceiver unit is set as the ground, a plus bias voltage is applied tothe LD, and also a negative bias voltage is applied to the PD. Thedisadvantage of the method is that the positive AC power applied to theLD is partially delivered to the PD connected to the negative powersource 19.

FIG. 3 is a circuit diagram of the optical transceiver module of FIG. 2.

Referring to FIG. 3, a light source L, an absorber A, and a lightreceiver D are diodes. Here, the light receiver D, which is also adiode, receives a reverse bias voltage. The light receiver D, to whichthe reverse bias voltage is applied, serves as a capacitor when the ACis supplied. Accordingly, an AC power input to the light source L ispartially delivered to a node 1 of a negative voltage through the diodeD to which the reverse bias voltage is applied. This phenomenon iscalled electrical crosstalk that reduces the sensitivity of a receiverdrastically.

When noise of −60 to −70 dBm is input to an input terminal of apreamplifier of the light receiver or the receiver, the receivergenerates an error. An electrical signal delivered to the light sourcehas a power larger than +10 dBm. Consequently, when more than −70 to −80dB of a signal applied to the light source is delivered to the receiver,the receiver generates an error. Especially, this problem is moreserious in manufacturing an optical transceiver using a single-substrateintegrated optical transceiver unit than in hybrid-integration of alight source and a receiver.

SUMMARY OF THE INVENTION

The present invention provides an optical transceiver module preventingelectrical crosstalk between a light source and a light receiver byusing a single-substrate optical transceiver unit when constituting themodule.

According to an aspect of the present invention, there is provided anoptical transceiver module including: an optical transceiver unitincluding a light source and a light receiver together integrated into asubstrate; a circuit unit including a drive circuit driving the lightsource and a detect circuit reading a signal of the light receiver; anda crosstalk prevention unit connected between the substrate and groundto prevent electrical crosstalk between the light source and the lightreceiver.

The crosstalk prevention unit may include a capacitor and a resistorrespectively connected between the substrate and the ground. An AC powerdelivered to the light receiver may be minimized by delivering an ACcurrent to the ground through the capacitor.

When connecting a resistor between the substrate and the ground, avoltage of the substrate may increase by a DC current applied to thelight source, and a reverse bias voltage may be applied to the lightreceiver even though a negative voltage is not applied to the lightreceiver. It may be desirable that the capacitor have a capacitance ofover 1 pF and most of the AC current flows to the ground.

A capacitor and a transistor respectively may be connected between thesubstrate and the ground to prevent the electrical crosstalk. Anabsorber may convert light emitted from the light source into a current,and then a current of the transistor may be controlled with a signal ofthe converted current. Additionally, the transistor may be used as acurrent source when the drive circuit of the light source includes adifferential pair circuit.

A gate resistor may be disposed between the gate and the ground when thetransistor is a MOS transistor, and the gate resistor is unnecessarywhen the transistor is a bipolar transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic view illustrating an optical transceiver unit inwhich an LD and a PD are integrated in an in-line manner into a singlesubstrate;

FIG. 2 illustrates a structure of an optical transceiver module usingthe optical transceiver unit of FIG. 1;

FIG. 3 is a circuit diagram of the optical transceiver module of FIG. 2;

FIG. 4 is a circuit diagram of an optical transceiver module accordingto an embodiment of the present invention; and

FIG. 5 is a circuit diagram of an optical transceiver module accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art. In thedrawings, the thicknesses of layers and regions are exaggerated forclarity. To facilitate understanding, identical reference numerals havebeen used, where possible, to designate identical elements that arecommon to the figures.

FIG. 4 is a circuit diagram of an optical transceiver module accordingto an embodiment of the present invention. Like in FIG. 3, an opticaltransceiver unit is a diode.

Referring to FIG. 4, the optical transceiver module includes an opticaltransceiver unit having a light source L1, an absorber A1, and a lightreceiver D1 formed on a substrate, a circuit unit 100, and a crosstalkprevention unit 200. Here, the light source L1 is a laser diode (LD),and the light receiver D1 is a photo-detector (PD). However, variouskinds of light sources and light receivers can be used.

The crosstalk prevention unit 200 includes a resistor 220 and acrosstalk prevention capacitor CT connected respectively between asubstrate and ground. The resistor 220 serves as a current path thatdirects a direct current (DC), which flows through the light source L1,to the ground. Thus, a positive voltage corresponding to the DC thatflows in the resistor 220 is formed in a substrate voltage line VB. Thepositive voltage, which is formed in the substrate voltage line VB,forms a reverse bias voltage in the absorber A1 and the light receiverD1. Accordingly, it is unnecessary to apply a negative voltage to thelight receiver D1. The crosstalk prevention capacitor C_(T) directs analternating current (AC), which flows in the light source L1, to theground. Thus, the AC does not reach an input terminal DO of apreamplifier through the light receiver D1 operating as a capacitor.

At this point, since an effective capacitance of the light receiver D1is 0.1-several pF, the crosstalk prevention capacitor C_(T) needs to besufficiently large, for example, several nF, such that the AC flowing tothe light source L1 is directed to the ground through the crosstalkprevention capacitor C_(T). In this way, an effect that (a portion of)an electrical power inputted into the light source L1 is delivered tothe light receiver D1 (i.e., electrical crosstalk) can be reduced.

The circuit unit 100 includes a drive circuit driving a light source anda detection circuit reading a signal of the light receiver D1.

The drive circuit of the circuit unit 100 is connected to the lightsource L1 and the absorber A1 to control a drive current of the lightsource L1. The drive circuit includes a controller 123 and an impedancematching resistor 122. The controller 123 is a voltage comparatorcomparing a voltage, which corresponds to a current flowing in theabsorber A1, to a reference voltage V2 and outputting a comparisonresult. The controller 123 includes a resistor 124 for generating DCcontrol voltage. Also, the controller 123 includes a capacitor C1 forbypassing the AC to ground. The controller capacitor C1 bypasses the ACoptical current, which is generated through the absorber A1, to theground. The impedance matching resistor 122 is used to adjust aneffective impedance of the input terminal LI to an appropriate size, forexample, 50Ω.

A DC drive current of the light source L1 is adjusted by feedback. Thatis, when the output intensity of the light source decreases due to anincrease of the surrounding temperature or characteristic deteriorationof the light source L1, light intensity absorbed in the absorber A1decreases, and thus a current flowing through the controller resistor124 decreases. Accordingly, a voltage applied to the controller resistor124 decreases and thus an output current of the controller 123increases. Consequently, the DC that flows in the light source L1increases, and also an output of the light source L1 increases.

Thus, an average output of the light source L1 is controlled through thefeedback of the controller 123.

Since the substrate voltage line VB is a positive voltage because of theresistor 220 according to the present invention, the detection circuitof the circuit unit 100 detecting a light receiving signal from thelight receiver is simple. That is, unlike in FIG. 3, a negative voltagesource is unnecessary and thus an inductor is unnecessary also. Theoutput terminal or a capacitor C_(O) in front of an input terminal DO ofa preamplifier is omitted for clarity.

According to the above embodiment, since the capacitor C_(T) and theresistor 220 are connected between the substrate voltage line VB andground, electrical crosstalk can be prevented and a reverse bias voltagecan be applied to the light receiver D1.

FIG. 5 is a circuit diagram of an optical transceiver module accordingto another embodiment of the present invention. An optical transceiverunit in the optical transceiver module is represented by diodes.

Referring to FIG. 5, the optical transceiver module includes an opticaltransceiver unit having a light source L1, an absorber A1, and a lightreceiver D1 formed on a substrate, a circuit unit 100a, and a crosstalkprevention unit 200a. Here, the light source L1 is an LD, and the lightreceiver D1 is a PD.

Unlike in FIG. 4, the circuit unit 100a further includes a light sourcedriver 137 and a preamplifier 138, and the crosstalk prevention unit220a includes a transistor Tr1 instead of a resistor 220.

An AC is prevented from being delivered to the light receiver D1 bybypassing it to ground through a crosstalk prevention capacitor C_(T1).In this structure, a DC supplied to the light source L1 is controlled bythe transistor Tr1 which receives an output of the controller 133. Anaverage optical current which flows through the absorber flows to thecontroller resistor 134. A voltage corresponding to the average opticalcurrent is applied to the controller 133. Additionally, the controllercapacitor C2 serves to bypass the AC optical current.

Since an output of the controller 133 is a current, the current flows toa gate resistor 240 and voltage corresponding to the current is appliedto a gate of the transistor Tr1. When the transistor Tr1 is an MOStransistor, the gate resistor 240 is required to generate the inputvoltage to a gate of the MOS transistor. However, when the transistorTr1 is a bipolar transistor, the gate resistor 240 is omitted. Thetransistor Tr1 can be used as a current source of a differential paircircuit including the light source driver 137, i.e., a current source ofa light source drive transistor Tr2. Like the resistor 122 of FIG. 4, aresistor 132 in front of the light source L1 is an impedance matchingresistor.

A conventional LD driver and trans-impedance preamplifier can be used inthe above structure of the optical transceiver module. The preamplifier138 is located behind the light receiver D1, and the light source driver137 is connected between an input terminal (not shown) and the impedancematching resistor 132. A complementary signal DATA of DATA is applied toan input of the light source drive transistor Tr2. When DATA is a ‘0’signal, a current of the light source drive transistor Tr2 is drainedsince a complementary signal of ‘0’ is ‘1’. Then, since a currentsupplied to the light source L1 decreases, and thus an output intensityof the light source L1 decreases, the light signal ‘0’ is outputted. Apower source resistor 141 is used to supply a current from aconventional power source.

Using the capacitor C_(T1) and the transistor Tr1, an electricalcrosstalk between the light source L1 and the light receiver D1 can beprevented, and a reverse bias voltage can be applied to the lightreceiver D1.

The optical transceiver module of the present invention can reduceelectrical crosstalk between the light source and the light receiver byconnecting the capacitor between the substrate and ground.

Additionally, since the reverse bias voltage can be applied to the lightreceiver by connecting the resistor or the transistor between thesubstrate and ground, it is unnecessary to apply a negative voltage tothe light receiver. Therefore, the optical transceiver module can besimply manufactured.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. An optical transceiver module, comprising: anoptical transceiver unit including a light source and a light receivertogether integrated into a substrate; a circuit unit including a drivecircuit driving the light source and a detection circuit reading asignal of the light receiver; and a crosstalk prevention unit connectedbetween the light source and the light receiver of the opticaltransceiver unit integrated in the substrate and ground for preventingelectrical crosstalk between the light source and the light receiver. 2.The optical transceiver module of claim 1, wherein the crosstalkprevention unit applies a reverse bias voltage to the light receiver. 3.The optical transceiver module of claim 1, wherein the crosstalkprevention unit is a capacitor.
 4. The optical transceiver module ofclaim 1, wherein the crosstalk prevention unit comprises a capacitor anda resistor respectively connected between the substrate and the ground.5. The optical transceiver module of claim 4, wherein the circuit unitincludes, in association with an absorber, monitoring light emitted fromthe light source, and a current supplied to the light source iscontrolled through feedback by the controller.
 6. The opticaltransceiver module of claim 1, wherein the crosstalk prevention unitcomprises a capacitor and a transistor respectively connected betweenthe substrate and the ground.
 7. The optical transceiver module of claim6, wherein the circuit unit further includes a controller, inassociation with an absorber, monitoring light emitted from a lightcontroller, and an on-off state of the transistor is controlled throughthe controller.
 8. The optical transceiver module of claim 6, whereinthe transistor is a MOS (metal oxide semiconductor) transistor and aresistor is placed between a gate of the MOS transistor and the ground.9. The optical transceiver module of claim 6, wherein the drive circuitincludes a differential pair circuit driving the light source, and thetransistor is used as a current source of the differential pair circuit.10. The optical transceiver module of claim 3, wherein the capacitor hasa capacitance of 1 pF or higher.
 11. The optical transceiver module ofclaim 1, wherein the optical transceiver comprises an absorber betweenthe light source and the light receiver.
 12. The optical transceivermodule of claim 1, wherein the light source is operated according to acurrent supply method and the light receiver is operated by a reversebias voltage.
 13. The optical transceiver module of claim 1, wherein thelight source is a laser diode and the light receiver is aphoto-detector.
 14. The optical transceiver module of claim 1, whereinthe substrate is an n-type semiconductor or a p-type semiconductor. 15.An optical transceiver module, comprising: an optical transceiver unitincluding a light source and a light receiver together integrated in asubstrate; and a crosstalk prevention unit connected between the lightsource and the light receiver of the optical transceiver unit integratedin the substrate and ground for preventing electrical crosstalk betweenthe light source and the light receiver.
 16. The optical transceivermodule of claim 15, wherein the crosstalk prevention unit applies areverse bias voltage to the light receiver.
 17. The optical transceivermodule of claim 15, wherein the crosstalk prevention unit comprises acapacitor connected between the substrate and the ground.
 18. Theoptical transceiver module of claim 17, wherein the capacitor has acapacitance of 1 pF or higher.
 19. The optical transceiver module ofclaim 15, wherein the crosstalk prevention unit comprises a capacitorand a resistor respectively connected between the substrate and theground.
 20. The optical transceiver module of claim 15, furthercomprising: a circuit unit that includes a detection circuit which readsa signal from the light receiver.
 21. The optical transceiver module ofclaim 20, wherein the circuit unit includes, in association with anabsorber, monitoring light emitted from the light source, and a currentsupplied to the light source is controlled through feedback by thecontroller.
 22. The optical transceiver module of claim 15, wherein thecrosstalk prevention unit comprises a capacitor and a transistorrespectively connected between the substrate and the ground.
 23. Theoptical transceiver module of claim 22, further comprising: a circuitunit that includes a controller, in association with an absorber,monitoring light emitted from a light controller, wherein an on-offstate of the transistor is controlled through the controller.
 24. Theoptical transceiver module of claim 22, wherein the transistor is a MOS(metal oxide semiconductor) transistor and a resistor is placed betweena gate of the MOS transistor and the ground.
 25. The optical transceivermodule of claim 22, further comprising: a drive circuit that includes adifferential pair circuit driving the light source, wherein thetransistor is used as a current source of the differential pair circuit.26. The optical transceiver module of claim 15, wherein the opticaltransceiver comprises an absorber between the light source and the lightreceiver.